1. Field of the Invention
The present invention relates to the isolation of equipment from transients which occur upon the opening or closing of switch contacts wherein the state of such switch contacts constitutes input information to the equipment. More specifically, this invention is directed to circuits for suppressing undesirable switch contact noise. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
2. Description of the Prior Art
It is common practice, particularly in process or equipment control applications, to employ mechanical or electromechanical switches to generate input information for data processing systems. As is well known, switch contacts usually do not close completely in one swing, a condition known as "bounce", and do not customarily open completely in one swing, a condition known as "chatter". As the contacts close and bounce and as the contacts open and chatter undesirable secondary signal fluctuations ae superimposed on the primary on-off signal. These secondary signal fluctuations, also known as noise or switching transients, may have a magnitude equal to that of the primary signal and can be incorrectly interpreted by the data processing system whereby an undesirable control action may be taken or an incorrect equipment status indication provided.
There have, in the prior art, been numerous techniques and apparatus proposed for isolating data processing equipment from switching transients. Such apparatus is generally known in the art as "switch debouncers". In their most basic form the prior art switch debouncers were passive devices and comprised merely a filter circuit interposed between the switch contacts and the equipment utilizing the switch state as input information. Such filter circuits, which may consist merely of a bypass capacitor or comprise a more complex form of low pass filter, cause large undesirable time delays between the switch closure and the application of a signal to the equipment responsive thereto. It has also been proposed to utilize multivibrator circuits interposed between the switch contacts and utilization equipment in order to isolate the contact noise from the electronic load. The prior art "active" switch debouncers which employed multivibrators to couple switch status change information to a load have been characterized by a number of inherent deficiencies. Thus, the prior art switch debouncers of the active type have been unsuitable for use with single pole single throw switches, have required plural voltage sources for operation and have been incapable of having their outputs connected to a common information bus.